Why Does OAuth v2 Have Both Access and Refresh Tokens?

Question

Section 4.2 of the draft OAuth 2.0 protocol indicates that an authorization server can return both an access_token (which is used to authenticate oneself with a resource) as well as a refresh_token, which is used purely to create a new access_token:

https://www.rfc-editor.org/rfc/rfc6749#section-4.2

Why have both? Why not just make the access_token last as long as the refresh_token and not have a refresh_token?

Answer 1

The idea of refresh tokens is that if an access token is compromised, because it is short-lived, the attacker has a limited window in which to abuse it.

Refresh tokens, if compromised, are useless because the attacker requires the client id and secret in addition to the refresh token in order to gain an access token.

Having said that, because every call to both the authorization server and the resource server is done over SSL - including the original client id and secret when they request the access/refresh tokens - I am unsure as to how the access token is any more "compromisable" than the long-lived refresh token and clientid/secret combination.

This of course is different to implementations where you don't control both the authorization and resource servers.

Here is a good thread talking about uses of refresh tokens: OAuth Archives.

A quote from the above, talking about the security purposes of the refresh token:

Refresh tokens... mitigates the risk of a long-lived access_token leaking (query param in a log file on an insecure resource server, beta or poorly coded resource server app, JS SDK client on a non https site that puts the access_token in a cookie, etc)

Answer 2

The link to discussion, provided by Catchdave, has another valid point (original, dead link) made by Dick Hardt, which I believe is worth to be mentioned here in addition to what's been written above:

My recollection of refresh tokens was for security and revocation. <...>

revocation: if the access token is self contained, authorization can be revoked by not issuing new access tokens. A resource does not need to query the authorization server to see if the access token is valid.This simplifies access token validation and makes it easier to scale and support multiple authorization servers. There is a window of time when an access token is valid, but authorization is revoked.

Indeed, in the situation where Resource Server and Authorization Server is the same entity, and where the connection between user and either of them is (usually) equally secure, there is not much sense to keep refresh token separate from the access token.

Although, as mentioned in the quote, another role of refresh tokens is to ensure the access token can be revoked at any time by the User (via the web-interface in their profiles, for example) while keeping the system scalable at the same time.

Generally, tokens can either be random identifiers pointing to the specific record in the Server's database, or they can contain all information in themselves (certainly, this information have to be signed, with MAC, for example).

How the system with long-lived access tokens should work

The server allows the Client to get access to User's data within a pre-defined set of scopes by issuing a token. As we want to keep the token revocable, we must store in the database the token along with the flag "revoked" being set or unset (otherwise, how would you do that with self-contained token?) Database can contain as much as len(users) x len(registered clients) x len(scopes combination) records. Every API request then must hit the database. Although it's quite trivial to make queries to such database performing O(1), the single point of failure itself can have negative impact on the scalability and performance of the system.

How the system with long-lived refresh token and short-lived access token should work

Here we issue two keys: random refresh token with the corresponding record in the database, and signed self-contained access token, containing among others the expiration timestamp field.

As the access token is self-contained, we don't have to hit the database at all to check its validity. All we have to do is to decode the token and to validate the signature and the timestamp.

Nonetheless, we still have to keep the database of refresh tokens, but the number of requests to this database is generally defined by the lifespan of the access token (the longer the lifespan, the lower the access rate).

In order to revoke the access of Client from a particular User, we should mark the corresponding refresh token as "revoked" (or remove it completely) and stop issuing new access tokens. It's obvious though that there is a window during which the refresh token has been revoked, but its access token may still be valid.

Tradeoffs

Refresh tokens partially eliminate the SPoF (Single Point of Failure) of Access Token database, yet they have some obvious drawbacks.

1. The "window". A timeframe between events "user revokes the access" and "access is guaranteed to be revoked".

2. The complication of the Client logic.

   without refresh token

   • send API request with access token
   • if access token is invalid, fail and ask user to re-authenticate

   with refresh token

   • send API request with access token
   • If access token is invalid, try to update it using refresh token
   • if refresh request passes, update the access token and re-send the initial API request
   • If refresh request fails, ask user to re-authenticate

I hope this answer does make sense and helps somebody to make more thoughtful decision. I'd like to note also that some well-known OAuth2 providers, including github and foursquare adopt protocol without refresh tokens, and seem happy with that.

Answer 3

Despite all the great answers above, I as a security master student and programmer who previously worked at eBay when I took a look into buyer protection and fraud, can say to separate access token and refresh token has its best balance between harassing user of frequent username/password input and keeping the authority in hand to revoke access to potential abuse of your service.

Think of a scenario like this. You issue user of an access token of 3600 seconds and refresh token much longer as one day.

1. The user is a good user, he is at home and gets on/off your website shopping and searching on his iPhone. His IP address doesn't change and have a very low load on your server. Like 3-5 page requests every minute. When his 3600 seconds on the access token is over, he requires a new one with the refresh token. We, on the server side, check his activity history and IP address, think he is a human and behaves himself. We grant him a new access token to continue using our service. The user won't need to enter again the username/password until he has reached one day life-span of refresh token itself.

2. The user is a careless user. He lives in New York, USA and got his virus program shutdown and was hacked by a hacker in Poland. When the hacker got the access token and refresh token, he tries to impersonate the user and use our service. But after the short-live access token expires, when the hacker tries to refresh the access token, we, on the server, has noticed a dramatic IP change in user behavior history (hey, this guy logins in USA and now refresh access in Poland after just 3600s ???). We terminate the refresh process, invalidate the refresh token itself and prompt to enter username/password again.

3. The user is a malicious user. He is intended to abuse our service by calling 1000 times our API each minute using a robot. He can well doing so until 3600 seconds later, when he tries to refresh the access token, we noticed his behavior and think he might not be a human. We reject and terminate the refresh process and ask him to enter username/password again. This might potentially break his robot's automatic flow. At least makes him uncomfortable.

You can see the refresh token has acted perfectly when we try to balance our work, user experience and potential risk of a stolen token. Your watch dog on the server side can check more than IP change, frequency of api calls to determine whether the user shall be a good user or not.

Another word is you can also try to limit the damage control of stolen token/abuse of service by implementing on each api call the basic IP watch dog or any other measures. But this is expensive as you have to read and write record about the user and will slow down your server response.

Answer 4

Neither of these answers get to the core reason refresh tokens exist. Obviously, you can always get a new access-token/refresh-token pair by sending your client credentials to the auth server - that's how you get them in the first place.

So the sole purpose of the refresh token is to limit the use of the client credentials being sent over the wire to the auth service. The shorter the TTL of the access-token, the more often the client credentials will have to be used to obtain a new access-token, and therefore the more opportunities attackers have to compromise the client credentials (although this may be super difficult anyway if asymmetric encryption is being used to send them). So if you have a single-use refresh-token, you can make the TTL of access-tokens arbitrarily small without compromising the client credentials.

Answer 5

To clear up some confusion you have to understand the roles of the client secret and the user password, which are very different.

The client is an app/website/program/..., backed by a server, that wants to authenticate a user by using a third-party authentication service. The client secret is a (random) string that is known to both this client and the authentication server. Using this secret the client can identify itself with the authentication server, receiving authorization to request access tokens.

To get the initial access token and refresh token, what is required is:

• The user ID
• The user password
• The client ID
• The client secret

To get a refreshed access token however the client uses the following information:

• The client ID
• The client secret
• The refresh token

This clearly shows the difference: when refreshing, the client receives authorization to refresh access tokens by using its client secret, and can thus re-authenticate the user using the refresh token instead of the user ID + password. This effectively prevents the user from having to re-enter his/her password.

This also shows that losing a refresh token is no problem because the client ID and secret are not known. It also shows that keeping the client ID and client secret secret is vital.

Answer 6

This answer has been put together by the help of two senior devs (John Brayton and David Jennes).

The main reason to use a refresh token is to reduce the attack surface.

Let's suppose there is no refresh key and let’s go through this example:

A building has 80 doors. All doors are opened with the same key. The key changes every 30 minutes. At the end of the 30 minutes I have to give the old key to the keymaker and get a new key.

If I’m the hacker and get your key, then at the end of the 30 minutes, I’ll courier that to the keymaker and get a new key. I’ll be able to continuously open all doors regardless of the key changing.

Question: During the 30 minutes, how many hacking opportunities did I have against the key? I had 80 hacking opportunities, each time you used the key (think of this as making a network request and passing the access token to identify yourself). So that’s 80X attack surface.

Now let’s go through the same example but this time let’s assume there’s a refresh key.

A building has 80 doors. All doors are opened with the same key. The key changes every 30 minutes. To get a new key, I can’t pass the old access token. I must only pass the refresh key.

If I’m the hacker and get your key, I can use it for 30 minutes, but at the end of the 30 minutes sending it to the keymaker has no value. If I do, then the keymaker would just say "This token is expired. You need to refresh the token." To be able to extend my hack I would have to hack the courier to the keymaker. The courier has a distinct key (think of this as a refresh token).

Question: During the 30 minutes, how many hacking opportunities did I have against the refresh key? 80? No. I only had 1 hacking opportunity. During the time the courier communicates with the keymaker. So that’s 1X attack surface. I did have 80 hacking opportunities against the key, but they are no good after 30 minutes.

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A server would verify an access token based on credentials and signing of (typically) a JWT.

An access token leaking is bad, but once it expires it is no longer useful to an attacker. A refresh token leaking is far worse, but presumably it is less likely. (I think there is room to question whether the likelihood of a refresh token leaking is much lower than that of an access token leaking, but that’s the idea.)

Point is that the access token is added to every request you make, whereas a refresh token is only used during the refresh flow So less chance of a MITM seeing the token

Frequency helps an attacker. Heartbleed-like potential security flaws in SSL, potential security flaws in the client, and potential security flaws in the server all make leaking possible.

In addition, if the authorization server is separate from the application server processing other client requests then that application server will never see refresh tokens. It will only see access tokens that will not live for much longer.

Compartmentalization is good for security.

Last but not least refresh tokens can get rotated. Meaning 'a new refresh token is returned each time the client makes a request to exchange a refresh token for a new access token.'. As refresh tokens are continually exchanged and invalidated, the threat is reduced. To give you an example: Tokens are usually expired after a TTL usually an hour.

Refresh tokens not always, but often are revoked upon usage and a new one issued. Meaning if you ever have a network failure, when you're retrieving the new refresh token, then the next time you send that refresh token, it's considered revoked and you have to sign in.

For more on rotation see here and here

Summary

• Reducing Frequency
• Compartmentalization
• Rotation (quicker invalidation) and more granular management (expiration time or number of requests made) of tokens.

All help to to mitigate threats

For another take on this see this awesome answer

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What refresh token is NOT about?

The ability to update/revoke access level through refresh tokens is a byproduct of choosing to use refresh tokens, otherwise a standalone access token could be revoked or have its access level modified when it expires and users gets a new token

Answer 7

This answer is from Justin Richer via the OAuth 2 standard body email list. This is posted with his permission.

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The lifetime of a refresh token is up to the (AS) authorization server — they can expire, be revoked, etc. The difference between a refresh token and an access token is the audience: the refresh token only goes back to the authorization server, the access token goes to the (RS) resource server.

Also, just getting an access token doesn’t mean the user’s logged in. In fact, the user might not even be there anymore, which is actually the intended use case of the refresh token. Refreshing the access token will give you access to an API on the user’s behalf, it will not tell you if the user’s there.

OpenID Connect doesn’t just give you user information from an access token, it also gives you an ID token. This is a separate piece of data that’s directed at the client itself, not the AS or the RS. In OIDC, you should only consider someone actually “logged in” by the protocol if you can get a fresh ID token. Refreshing it is not likely to be enough.

For more information please read http://oauth.net/articles/authentication/

Answer 8

Clients can be compromised in many ways. For example a cell phone can be cloned. Having an access token expire means that the client is forced to re-authenticate to the authorization server. During the re-authentication, the authorization server can check other characteristics (IOW perform adaptive access management).

Refresh tokens allow for a client only re-authentication, where as re-authorize forces a dialog with the user which many have indicated they would rather not do.

Refresh tokens fit in essentially in the same place where normal web sites might choose to periodically re-authenticate users after an hour or so (e.g. banking site). It isn't highly used at present since most social web sites don't re-authenticate web users, so why would they re-authenticate a client?

Answer 9

To further simplify B T's answer: Use refresh tokens when you don't typically want the user to have to type in credentials again, but still want the power to be able to revoke the permissions (by revoking the refresh token)

You cannot revoke an access token, only a refresh token.

Answer 10

Why not just make the access_token last as long as the refresh_token and not have a refresh_token?

In addition to great answers other people have provided, there is another reason why we would use refresh tokens and it's to do with claims.

Each token contains claims which can include anything from the user's name, their roles, or the provider which created the claim. As a token is refreshed, these claims are updated.

If we refresh the tokens more often, we are obviously putting more strain on our identity services; however, we are getting more accurate and up-to-date claims.

Answer 11

Assume you make the access_token last very long, and don't have refresh_token, so in one day, hacker get this access_token and he can access all protected resources!

But if you have refresh_token, the access_token's live time is short, so the hacker is hard to hack your access_token because it will be invalid after short period of time. Access_token can only be retrieved back by using not only refresh_token but also by client_id and client_secret, which hacker doesn't have.

Answer 12

While refresh token is retained by the Authorization server. Access token are self-contained so resource server can verify it without storing it which saves the effort of retrieval in case of validation. Another point missing in discussion is from rfc6749#page-55

"For example, the authorization server could employ refresh token rotation in which a new refresh token is issued with every access token refresh response.The previous refresh token is invalidated but retained by the authorization server. If a refresh token is compromised and subsequently used by both the attacker and the legitimate client, one of them will present an invalidated refresh token, which will inform the authorization server of the breach."

I think the whole point of using refresh token is that even if attacker somehow manages to get refresh token, client ID and secret combination. With subsequent calls to get new access token from attacker can be tracked in case if every request for refresh result in new access token and refresh token.